The present system and method relate generally to hybrid motor vehicles, and more particularly, to a hybrid powertrain system adapted for installation in a hybrid motor vehicle.
Automobile manufacturers are constantly working to improve fuel efficiency in motor vehicles. Improvements in fuel efficiency are typically directed toward reducing weight, improving aerodynamics, and reducing power losses through the vehicle powertrain. However, the need to improve fuel efficiency is commonly offset by the need to provide enhanced comfort and convenience to the vehicle operator. As an example, manually-shifted transmissions are more fuel efficient than automatic transmissions due to lower parasitic losses. The higher losses associated with conventional automatic transmissions originate in the torque converter, the plate clutches and the hydraulic pump used to control operation of the hydraulic shift system. However, a vast majority of domestic motor vehicles, for example, are equipped with automatic transmissions due to the increased operator convenience they provide. Recent advances in power-operated shift systems have allowed development of “automated” versions of manual transmissions, which automatically shift between sequential gear ratios without any input from the vehicle operator. Thus, automated manual transmissions provide the convenience of a traditional automatic transmission with the efficiency of a manual transmission.
Passenger vehicle and heavy truck manufacturers are also actively working to develop alternative powertrain systems in an effort to reduce the level of pollutants exhausted into the air by conventional powertrain systems equipped with internal combustion engines. Significant development efforts have been directed to electric and fuel-cell vehicles. Unfortunately, these alternative powertrain systems suffer from several disadvantages and, for all practical purposes, are still under development. However, “hybrid” electric vehicles, which include an internal combustion engine and an electric or hydraulic motor, offer a compromise between traditional internal combustion engine powered vehicles and full electric powered vehicles. These hybrid vehicles are equipped with an internal combustion engine and an electric or hydraulic motor that can be operated independently or in combination to provide motive power to the vehicle.
There are two types of hybrid vehicles, namely, series hybrid and parallel hybrid vehicles. In a series hybrid vehicle, power is delivered to the wheels by the electric motor, which draws electrical energy from a generator or a battery. The engine is used in series hybrid vehicles to drive a generator that supplies power directly to the electric motor or charges the battery when the state of charge falls below a predetermined value. In parallel hybrid vehicles, the electric motor and the engine can be operated independently or in combination pursuant to the running conditions of the vehicle.
Typically, the control strategy for such parallel hybrid vehicles utilizes a low-load mode where only the electric motor is used to drive the vehicle, a high-load mode where only the engine is used to drive the vehicle, and an intermediate assist mode where the engine and electric motor are both used to drive the vehicle. However, prior art parallel hybrid powertrain systems are relatively inefficient at transitioning from one mode to another, particularly the transition from low-load mode to high-load mode. Furthermore, a majority of prior art hybrid powertrain systems are designed for use in passenger vehicles that employ a relatively light duty gasoline or diesel engine, as opposed to the relatively heavy duty diesel engines found in over-the-road trucks. While hybrid powertrain systems employing a light duty gasoline or diesel engine may be readily transitioned from one operating mode to another without any perceived transition event by the vehicle operator, prior art powertrain systems employing a heavy duty diesel engine are notoriously rough during the transition from one operating mode to another, particularly when the diesel engine is started. Accordingly, there exists a need for improved hybrid powertrain systems that facilitate an efficient and smooth transition from one operating mode to another, particularly in vehicles that employ a heavy duty diesel engine.